The present invention relates to an arrangement for propelling an aircraft comprising a jet engine and an outlet nozzle arranged downstream of the jet engine, the jet engine being of a type which generates an internal core flow and an external fan flow, a part of the outlet nozzle having an internal surface which defines a gas duct for the fan flow. The invention also relates to a correspondingly executed outlet nozzle.
The expression “core flow” is used here to denote a radial inner gas flow which originates from the combustion chamber of the jet engine. The core flow accordingly has a high energy content. The expression “fan flow” is used here to denote a radial outer gas flow (air flow) which flows in an outer duct in the jet engine, which extends from a front fan in the jet engine and past the combustion chamber. The fan flow is also known as the bypass flow. The jet engine is thus of the double-flow type.
Previously disclosed is the protection of an aircraft against possible attack by giving the aircraft a low so-called signature. The expression signature in this context denotes contrast against the background. An aircraft should, for example, have a low signature in respect of infrared radiation (IR) and radar. Hot structures and warm exhaust gases give rise to an IR signature. Metallic' surfaces of the jet engine, such as turbine parts, can give rise to an IR signature when they become hot during operation of the jet engine.
The present invention is intended for a propulsion arrangement for an aircraft that is designed to achieve a low signature. One way of achieving a low IR signature is to execute the outlet nozzle so that the aforementioned internal parts of the jet engine, which are hot during operation, are at least substantially concealed from view from the rear through the outlet of the outlet nozzle.
The core flow and the fan flow emerging from the jet engine move in a direction essentially parallel to the axial direction of the jet engine. According to a previously disclosed outlet nozzle, its gas duct has a form which curves in relation to the axial direction of the jet engine in such a way that the aforementioned parts of the jet engine that are hot during operation are concealed from view from the rear. According to another previously disclosed outlet nozzle, the cross section of the gas duct is transformed from a circular form at the inlet to the outlet nozzle into an elongated elliptical form at the outlet from the outlet nozzle. According to these examples, the axial gas flow emerging from the jet engine will be affected.
In an outlet nozzle having such a form of the gas duct, cold and hot bands can appear on the external metallic surfaces of the outlet nozzle due to the fact that the hot core flow displaces the fan flow in the area of the curve or the change in the cross-sectional form. Such hot bands lead to an increased risk of detection as a consequence of an increased IR signature. Cold and hot bands can also give rise to thermal stresses in the nozzle.
It is desirable to reduce any infrared radiation given off by the aircraft during flight, which remedies the above problem of cold and hot bands on the surfaces of the outlet nozzle.
This is achieved, therefore, with an arrangement for propelling an aircraft comprising a jet engine and an outlet nozzle arranged downstream of the jet engine, the jet engine being of a type which generates an internal core flow and an external fan flow, a part of the outlet nozzle having an internal surface which defines a gas duct for the fan flow, characterized in that the said part of the outlet nozzle comprises a wall structure that is arranged at a distance from the internal surface such that it separates the gas duct for the fan flow from an internal gas duct for the core flow.
The above-mentioned problem of cold and hot bands on the surfaces of the outlet nozzle is particularly pronounced when the internal surface exhibits a form such that at least one part of the fan flow is deflected.
According to a preferred embodiment, the wall structure is tubular and has an extent in the axial direction of the outlet nozzle. The wall structure is preferably also of the single-wall type, appropriately having the character of an internal shell or sleeve.
According to another preferred embodiment, the wall structure comprises at least one through hole from the gas duct for the fan flow to the gas duct for the core flow for the purpose of cooling the wall structure.
With a hole configuration of this kind, the temperature of the wall structure is reduced and with it its IR signature.
According to a further development of the previous embodiment, the wall structure is so arranged that a cross-sectional area through the gas duct for the fan flow reduces successively in the direction of the outlet from the outlet nozzle. The quantity of fan air that is forced into the cooling openings in the wall structure is increased in this way, with an associated further reduction in the temperature and the IR signature of the wall structure.
According to another preferred embodiment, the wall structure is designed to absorb thermal loadings, and a structural part which comprises the internal surface that defines the fan duct is designed to absorb mechanical loadings during operation. This offers possibilities for utilizing lighter materials, such as composites, in the external wall/sleeve of the outlet nozzle.
This is achieved, therefore, with an outlet nozzle for a jet engine arranged for positioning downstream of the jet engine, the outlet nozzle comprising a duct for conducting a gas emerging from the jet engine, which duct exhibits a gas inlet and a gas outlet, in conjunction with which one part of the outlet nozzle has an internal surface which defines the gas duct, characterized in that the aforementioned part of the outlet nozzle comprises a wall structure that is arranged at a distance from the internal surface such that it separates an external gas duct in a radial direction for a fan flow emerging from the jet engine from an inner gas duct in a radial direction for a core flow emerging from the jet engine.
The above-mentioned problem of cold and hot bands on the surfaces of the outlet nozzle is particularly pronounced when the internal surface exhibits a form such that at least a proportion of the gas emerging from the jet engine is deflected.
Additional preferred embodiments and advantages of these can be appreciated from the following description, the figures and the claims.